The present invention relates generally to audio signal amplifier circuits and more particularly to audio signal amplifier circuits used for audio signal processing circuits in, for instance, audio equipment, televisions and VTRs, and to an audio signal amplifier circuit in the form of a one-chip integrated circuit having a muting function.
In an audio signal amplifier circuit that employs transistors and the like, unpleasant sound like so-called pop sound ensues when a power switch is turned on and if the worst comes to the worst, a speaker connected to a power amplifier may be damaged. It is therefore the practice generally followed to insert a mute circuit in order to compulsorily cut or ground a path for an audio signal until the power amplifier and the like arrive at a steady stable state.
Referring to a block diagram of FIG. 2, the construction and operation of a specific audio signal amplifier circuit having such a mute circuit will be described.
Numeral 1 denotes an audio signal amplifier circuit which is normally an integrated circuit and comprises an amplifier 2, a mute circuit 3, a switch circuit SW2 and the like. On receiving an audio input signal I, the amplifier 2 amplifies the signal and outputs it as an audio output signal A. The mute circuit 3 receives a mute input signal Min generated (a generator circuit is not shown) to suppress an audio signal, depending on the condition of a power supply circuit or the switch input, and produces a mute control signal M accordingly. The mute control signal M is supplied to an IC terminal 1a and the switch circuit SW2. On receiving the mute control signal M, the switch circuit SW2 cuts the signal line of the audio input signal I. Moreover, a switch circuit SW1 provided outside the integrated circuit grounds the signal line of the audio output signal A on receiving the mute control signal M via the terminal 1a. In this way, a muting function is fulfilled by compulsorily cutting both the input and output sides of the audio signal or reducing them to a reference potential when the conventional audio signal amplifier circuit receives the mute input signal Min.
In the conventional audio signal amplifier circuit, the switch circuit SW1 has thus been arranged as a discrete part but not incorporated in the audio signal amplifier circuit in the form of an integrated circuit. For this reason, the mute control signal M has to be sent to the switch circuit SW1 and one IC terminal is specifically allotted for the purpose.
However, any increase in the number of IC terminals is undesirable because it is against a demand for reducing the IC size and the apparatus itself as well.
Since one mute control signal is used simultaneously to switch two of the switch circuits SW1, SW2, in the case of the operating times of the switch circuits being varied, the switch circuit SW1 of the output side may fail to suppress the noise produced by switching of the switch circuit SW2 of the input side. Accordingly, the muting function is not guaranteed to perform in stability. In other words, the problem is that more man-hours are needed for adjustment and so forth.